<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>进入网吧案例</title>
    <script>
        var age = prompt('请输入您的年龄')
        if (age < 18) {
            alert('您不能进入网吧')
        } else {                               //else后面直接跟花括号没有小括号
            alert('您可以进入网吧')
        }
    </script>
</head>

<body>

</body>

</html>